1// Copyright ©2019 The Gonum Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style
3// license that can be found in the LICENSE file.
4
5package gonum
6
7// Dcombssq adds two scaled sum-of-squares quantities, V := V1 + V2,
8//  V_scale^2 * V_ssq := V1_scale^2 * V1_ssq + V2_scale^2 * V2_ssq
9// and returns the result V.
10//
11// Dcombssq is an internal routine. It is exported for testing purposes.
12func (Implementation) Dcombssq(scale1, ssq1, scale2, ssq2 float64) (scale, ssq float64) {
13	if scale1 >= scale2 {
14		if scale1 != 0 {
15			return scale1, ssq1 + (scale2/scale1)*(scale2/scale1)*ssq2
16		}
17		// If the input is non-negative and we are here, then scale2 must inevitably be 0, too.
18		return 0, 0
19	}
20	return scale2, ssq2 + (scale1/scale2)*(scale1/scale2)*ssq1
21}
22